1. Field of the Invention
The present invention pertains to the field of circuit design. More particularly, this invention relates to voltage controlled resistors.
2. Background
A voltage controlled resistor (VCR) is a circuit which provides a variable resistance based on a voltage input. VCRs can be used in a wide variety of analog and digital applications. One advantage to VCRs is that a single VCR can be used to provide a range of resistances, thereby allowing system designers a great deal of flexibility.
One use for VCRs is in microprocessors and other synchronous integrated circuit (IC) devices. These synchronous devices typically depend on a clock signal to operate. The clock signal provides a control signal so that the components within the device can perform their functions in a synchronized manner. Typically, these synchronous devices require a constant clock signal in order to operate properly. Without an adequate and constant clock signal, these devices operate either significantly below their potential or not at all. Thus, it would be beneficial to provide a circuit which provides a constant clock signal.
Additionally, different devices require clock signals of different frequencies. These different frequency requirements are due to a wide variety of factors, including the maximum operating speed of components within the device, the maximum operating speed of other devices which are to be coupled to this device, desired power dissipation of the device, the maximum desired operating temperature of the device, etc. Building separate clock generators which operate at each of these different frequencies, however, has several disadvantages. One such disadvantage is the actual building and maintaining of a large number of clock generating devices. A different design is used for each of the different clock generating devices, which results in a different fabrication process as well. Another disadvantage is ensuring that a supply of the proper frequency generators is available for those who need them, while at the same time not having a supply that greatly exceeds the demand. Thus, it would be beneficial to provide a single controllable clock signal generating device which can be adjusted to generate a wide range of frequencies.
One method which has been used to generate a clock signal uses a phase locked loop including a voltage controlled oscillator. The voltage controlled oscillator includes multiple delay stages which include a voltage controlled resistance. The voltage controlled resistance uses a single voltage control input to adjust the resistance of the delay stage. The higher the resistance of the delay stage, the longer the delay imposed by the stage. And, the longer the delay imposed by the stage, the lower the frequency of oscillation.
These voltage controlled resistances provide some control over the frequency of the clock signal being generated. One such voltage controlled resistance circuit is shown in FIG. 1. Circuit 100 of FIG. 1 includes a control input voltage 110 which controls the current at output 120. The larger the control input voltage 110, the greater the current at output 120. Thus, increasing the input voltage decreases the resistance of circuit 100. FIG. 2 is a graph showing the I-V characteristic of the circuit of FIG. 1 at output 120 for control input voltages of 3 v, 2 v, and 1.5 v. As shown in FIG. 2, larger control input voltages can provide larger output currents. Therefore, by changing the control input voltage, the delay stages of the oscillator can provide different resistances, which results in a range of clock frequencies which can be generated.
Larger control input voltages are necessary in FIG. 1 to obtain the largest range of resistances. However, circuit 100 is also very susceptible to noise in the voltage control input line at the larger control input voltages. That is, slight changes in the input control voltage line can result in noticeable changes in the output current. Thus, the frequency of the oscillator can be noticeably affected by slight changes in the input control voltage. Such susceptibility to noise is undesirable in a clock generator because it provides a changing clock signal rather than a constant clock signal. Thus, it would be beneficial to provide a delay stage for a voltage controlled oscillator which reduces the impact that noise in the voltage input line has on the output frequency, while at the same time maintaining a wide range of frequencies.
The present invention provides for these and other advantageous results.